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Nitrogen fertilisation of grasslands and dairy cow nutrition. Consequences on N losses

INRA Prod. Anim., 13(1), 61-72.


INRA Unité Mixte de Recherches sur la Production du Lait, 35590 Saint Gilles


The changes in agricultural policy and the increased concern about the effects of intensive production systems on the natural environment may lead to a reduction in the levels of nitrogen (N) fertilisation on grassland. The aim of this review is to quantify the consequences of lowering N fertilisation on dairy cow nutrition.

For grass species harvested at the same age of regrowth, a reduction in N fertilisation leads to a 0.02 unit decrease in organic matter (OM) digestibility, but the site of OM digestion is unaffected. Despite a much lower crude protein (CP) content in poorly fertilised grass, the amount of non-ammonia nitrogen entering the intestine is decreased by only about 5 % since the efficiency of microbial protein synthesis is unaffected. The rumen degradability of CP is, however, slightly decreased. Reducing N fertilisation could reduce NEL by 0.3 MJ kg-1 DM and Metabolisable Protein (MP) by 5 to 12 g kg-1 DM. These moderate effects can be attributed to the fact that any decrease in CP content is compensated for by an increase in water-soluble carbohydrates, which are completely digestible and provide a readily available source of energy for ruminal proteosynthesis. Conversely, cell wall content remains unchanged. On the average, N fertilisation has no effect on the quantity of dry matter voluntarily ingested by stall-feeding animals, but herbage intake at grazing may be reduced since low amounts of N fertilisation might reduce the ease of prehension of herbage by reducing the green leaf mass per unit area. Lowering the levels of N fertilisation would appear to be an efficient means of reducing N loss in ruminants. The possible consequences on dairy cow nutrition are discussed.

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